Hydrogen absorbing alloy powder and process for producing same

ABSTRACT

The invention provides a hydrogen absorbing alloy electrode obtained by the step P 1  of preparing a hydrogen absorbing alloy powder containing cobalt and nickel, the step P 2  of subjecting the surfaces of the alloy particles to a reduction treatment with high-temperature hydrogen by holding the powder in a high-temperature hydrogen atmosphere under the conditions of temperature, pressure and time sufficient to reduce oxides formed in a surface layer portion of each of the alloy particles, not melting the alloy particles and not permitting the alloy particles to absorb hydrogen, the step P 3  of treating the resulting powder with an acid or alkali by immersing the powder in an acid or alkaline aqueous solution, followed by suction filtration, washing with water and drying, and the step P 4  of applying the resulting power to an electrically conductive substrate and shaping the substrate in the form of the electrode. The electrode thus provided has higher activity than conventionally.

This is a Division of application Ser. No. 09/147,482 filed Jan. 7, 1999now U.S. Pat. No. 6,238,822, which is a 371 of PCT/JP98/00324, filedJan. 26, 1998. The disclosure of the prior application(s) is herebyincorporated by reference in its entirety.

TECHNICAL FIELD

The present invention relates to a hydrogen absorbing alloy powder foruse as a material for electrodes (negative electrodes) of metallicoxide-hydrogen batteries such as nickel-hydrogen batteries, and aprocess for producing the powder, and more particularly to the surfacetreatment of a hydrogen absorbing alloy powder.

BACKGROUND ART

Hydrogen absorbing alloy electrodes serving as the negative electrodesof nickel-hydrogen batteries are prepared by pulverizing a hydrogenabsorbing alloy ingot to obtain a hydrogen absorbing alloy powder,admixing a binder with the powder and shaping the mixture in the form ofthe electrode. The hydrogen absorbing alloys heretofore developedinclude those of AB₅-type rare earths having a crystal structure of theCaCu₅, type, such Mm—Ni alloys, and TiNi₂ alloys having a Laves-phasestructure of the C14-type or C15-type.

With the nickel-hydrogen batteries having a hydrogen absorbing alloyelectrode as the negative electrode, a gas-phase reaction and anelectrochemical reaction proceed at the same time on the surface of thehydrogen absorbing alloy owing to the contact of the alloy surface withan alkaline electrolyte. More specifically, in the relationship betweenthe hydrogen pressure and the temperature, hydrogen is absorbed by thealloy, or the alloy desorbs hydrogen (gas-phase reaction). In thevoltage-current relationship, on the other hand, application of voltage(charging) permits the alloy to absorb the hydrogen produced by theelectrolysis of water, and the delivery of current (discharging)oxidizes hydrogen to form water (electrochemical reaction). Theproperties of the alloy surface are therefore important in improving theperformance of the nickel-hydrogen battery.

Accordingly, to improve the activity of the hydrogen absorbing alloy foruse in nickel-hydrogen batteries, it is conventional practice to immersea hydrogen alloy powder in an aqueous acid solution for surfacetreatment as disclosed in JP-B-225975/1993, or in an aqueous alkalinesolution for surface treatment as disclosed in JP-B-175339/1988. Thesurface treatment removes an oxide film formed in the surface layerportions of the alloy particles, permitting rare-earth elements (such asLa) to dissolve out and forming a nickel- or cobalt-rich layer in thesurfaces layer portions of the particles, whereby the alloy is givenimproved electrochemical catalytic activity.

However, we have found that the conventional surface treatment stillfails to afford sufficient activity although forming the nickel- orcobalt-rich layer in the surface layer portions of the alloy particles.

An object of the present invention is to provide a hydrogen absorbingalloy powder having higher activity than conventionally, a process forproducing the powder, a hydrogen absorbing alloy electrode wherein thepower is used, and a metallic oxide-hydrogen battery comprising theelectrode.

DISCLOSURE OF THE INVENTION

In producing a hydrogen absorbing alloy powder of the present invention,a starting hydrogen absorbing alloy powder containing nickel and cobaltis held in a high-temperature hydrogen atmosphere under the conditionsof temperature, pressure and time sufficient to reduce oxides formed ina surface layer portion of each of the alloy particles 1, not meltingthe alloy particles 1 and not permitting the alloy particles to absorbhydrogen, and thereafter surface-treated with an acid or alkalinetreating liquid. In this process, the temperature is in the range of100° C. to 900° C., the pressure is in the range of 1 atm to 3 atm, andthe time is in the range of 30 minutes to 10 hours. The acid treatingliquid is, for example, a hydrochloric acid solution. The alkalinetreating liquid to be used is at least one aqueous solution selected,for example, from among aqueous solution of KOH, aqueous solution ofNaOH and aqueous solution of LiOH.

The hydrogen absorbing alloy powder obtained by the above productionprocess is applied to an electrically conductive substrate and shaped inthe form of an electrode to prepare a hydrogen absorbing alloy electrodeof the invention.

The oxide film formed in the surface layer portions of the alloyparticles 1 in the step of preparing the starting hydrogen absorbingalloy powder is reduced by the high-temperature hydrogen atmosphere(reduction treatment with high-temperature hydrogen) of the aboveprocess and thereby converted to a first metal-rich layer 3 which isenriched in metals (nickel and cobalt). Since the temperature, pressureand time for the treatment are adjusted to the respective rangesmentioned, the oxide film is fully reduced without the likelihood of thealloy particles 1 melting or absorbing hydrogen.

The alloy powder is thereafter subjected to a surface treatment with theacid or alkaline treating liquid, whereby oxides of rare-earth elements(such as La), or Al, etc. are allowed to dissolve out from a surfacelayer portion of the first metal-rich layer 3. A second metal-rich layer4 further enriched in the metals (nickel and cobalt) is formed in thesurface layer portion of the first metal-rich layer 3. The firstmetal-rich layer 3 is internally studded with relatively small clusters30 of the metals (nickel and cobalt), while the second metal-rich layer4 is visually found to be internally studded with many relatively largeclusters 40 of the metals (nickel and cobalt).

According to the present invention, the first metal-rich layer 3 formedby the reduction treatment with high-temperature hydrogen and enrichedin nickel and cobalt is treated with an acid or alkali to form thesecond metal-rich layer 4 which is further enriched in nickel andcobalt. The invention therefore affords higher activity than the priorart wherein an acid treatment or alkali treatment only is conducted.

The hydrogen absorbing alloys usable according to the invention arethose having a crystal structure of the CaCu₅ type, and alloys having aLaves-phase structure of the C14-type or C15-type. Preferable to use arealloys having a crystal structure of the CaCu₅ type.

Examples of useful alloys having the CaCu₅-type crystal structure arethose represented by MmNi₂CoAlMn and obtained by substituting the La ofLaNi₅ with Mm (misch metal) which is a mixture of rare-earth elements,i.e., alloys represented by the formula MmNi_(x)M1_(y)M2_(z) (wherein Mmis a mixture of rare-earth elements, M1 is at least one element selectedfrom among Co, Al and Mn, M2 is a transition metal different from M1, xis a positive real number, x, y and z are such that 4.7≦x+y+z≦5.4).

Examples of useful alloys having a Laves-phase structure are thoserepresented by AB₂ (wherein A is at least one of Ti and Zr, and B is atleast one element selected from among Ni, Co, V, Mn, Fe and Cr). Morespecifically, TiNi₂ and Ti_(0.5)Zr_(0.5)Ni₂ are useful.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a scheme showing the process of the invention for producing ahydrogen absorbing alloy electrode;

FIG. 2 is a sectional view showing an alkaline battery;

FIG. 3 is a diagram for illustrating the effect of a reduction treatmentof the invention with high-temperature hydrogen;

FIG. 4 is a diagram showing the structure of surface layer portion of aparticle of hydrogen absorbing alloy embodying the invention;

FIG. 5 is a table showing the results obtained by analyzing the surfacelayer portion of the alloy particle to determine the compositionsthereof;

FIG. 6 is a perspective view partly broken away and showing theconstruction of a test cell;

FIG. 7 is a table showing the results of tests conducted to substantiatethe advantage of the invention; and

FIG. 8 is another table showing the results of the tests.

BEST MODE OF CARRYING OUT THE INVENTION

FIG. 2 shows the construction of a nickel-hydrogen battery (for example,1000 mAh in battery capacity) of AA size and of the type wherein thepositive electrode is dominant, and the present invention is to bepracticed. The illustrated battery, which is an alkaline battery, has aclosed construction comprising a positive electrode 11, negativeelectrode 12, separator 13, positive electrode lead 14, negativeelectrode lead 15, external positive terminal 16, can 17 serving also asa negative terminal, closure 18, etc. The positive electrode 11 and thenegative electrode 12 are accommodated, as rolled up in a spiral formwith the separator 13 interposed therebetween, in the can 17. Thepositive electrode 11 is connected by the lead 14 to the closure 18, andthe negative electrode 12 by the lead 15 to the can 17. An insulatingpacking 20 is provided at the junction of the can 17 and the closure 18to seal off the battery. A coiled spring 19 is interposed between theexternal positive terminal 16 and the closure 18. The spring 19 iscompressed to release a gas from inside the battery to the atmospherewhen the internal pressure of the battery builds up abnormally.

A hydrogen absorbing alloy electrode for use as the negative electrode12 is produced by the steps shown in FIG. 1.

First, a hydrogen absorbing alloy powder is prepared as specified incomposition and particle size (step P1). For example, Mm, Ni, Co, Al andMn are mixed together in the mole ratio of 1.0:3.1:1.0:0.3:0.6, and themixture is melted in an arc melting furnace having an argon atmosphereand thereafter allowed to cool spontaneously to obtain an ingot ofhydrogen absorbing alloy represented by the formulaMmNi_(3.1)CoAl_(0.3)Mn_(0.6). The ingot is mechanically pulverized inthe air to obtain a hydrogen absorbing alloy powder having a meanparticle size of 80 micrometers.

Next, the alloy powder is placed into a heat-resistantpressure-resistant container of stainless steel and heated at 300° C.after evacuation, hydrogen gas is then introduced into the container to1.2 atm, and the powder is held in this state for 30 minutes. In thisway, the alloy particles are subjected to a surface treatment (reductiontreatment with high-temperature hydrogen, step P2).

The temperature, pressure and time for the reduction treatment withhigh-temperature hydrogen are not limited to the above values but are sodetermined that the oxide film formed in the surface layer portions ofthe alloy particles as will be described below can be fully reducedwithout permitting the particles to melt and to absorb hydrogen. Thetreatment is conducted, for example, at a temperature in the range of100° C. to 900° C. and at a pressure in the range of 1.0 atm to 3.0 atmfor 30 minutes to 10 hours.

Subsequently, the alloy powder resulting from the reduction treatment isimmersed in an acid aqueous solution, for example, in a 0.5Nhydrochloric acid solution (room temperature), at a pH of 0.3 for 2hours, followed by suction filtration, washing with water and drying,whereby the powder is subjected to an acid treatment (step P3).

The acid aqueous solution is not limited to the hydrochloric acidsolution; an aqueous solution having a strong acidity of 0.3 to 2.0 inpH can be used. For example, a sulfuric acid solution or nitric acidsolution is usable. In view of the battery characteristics, thehydrochloric acid solution is more preferable than the sulfuric acidsolution or nitric acid solution in that the aqueous solution is freefrom the sulfate radial (So₄ ²⁻) or nitrate radical (NO₃ ³¹ ).

The hydrogen absorbing alloy powder can be treated with an alkalineaqueous solution instead of the acid treatment. The alloy powder isimmersed, for example, in a 30 wt. % aqueous solution of potassiumhydroxide (80° C.), useful as an electrolyte, for 2 hours, followed bysuction filtration and drying. The alkaline aqueous solution is notlimited to the aqueous solution of potassium hydroxide but can be astrongly alkaline aqueous solution consisting predominantly of potassiumhydroxide (KOH), such as an aqueous solution of KOH and LiOH, aqueoussolution of KOH and NaOH or aqueous solution of KOH, NaOH and LiOH. Anaqueous solution of LiOH and NaOH is also usable.

The hydrogen absorbing alloy powder resulting from the acid treatment isthereafter mixed with a 5 wt. % aqueous solution of a binder such as PEO(polyethylene oxide) in the ratio of 100:20 by weight to prepare apaste, which is applied to opposite surfaces of a substrate of punchingmetal plated with nickel, followed by drying at room temperature andcutting to a predetermined size, whereby a hydrogen absorbing alloyelectrode is produced (step P4).

The electrode thus obtained is incorporated as the negative electrodeinto the nickel-hydrogen battery shown in FIG. 2. A sintered nickelelectrode is usable as the positive electrode, an alkali-resistantnonwoven fabric as the separator, and a 30 wt. % aqueous solution ofpotassium hydroxide as the electrolyte.

In the process shown in FIG. 1 for producing the hydrogen absorbingalloy electrode, the surfaces of the alloy particles as prepared by stepP1 come into contact with the air or the water in the air, with theresult that an oxide film 2 of nickel oxide and cobalt oxide is formedin the surface layer portions of the alloy particles 1 as shown in FIG.3, (a).

The oxide film 2 is thereafter reduced with high-temperature hydrogen instep P2 and thereby converted to a first metal-rich layer 3 which isenriched in nickel and cobalt as shown in FIG. 3, (b).

Further in step P3, rare-earth elements such as La dissolve out from asurface layer portion of the first metal-rich layer 3, with the resultthat a second metal-rich layer 4 further enriched in nickel and cobaltis formed in the surface layer portion of the first metal-rich layer 3.

FIG. 4 is a diagram schematically showing the surface layer portion ofthe alloy particle obtained by the above process, as observed under atransmission electron microscope. A sample was prepared by ion etchingfor the observation of the surface layer portion.

As illustrated, the first metal-rich layer 3 is internally studded withrelatively small clusters 30 of nickel and cobalt, while the secondmetal-rich layer 4 is internally studded with many relatively largeclusters 40 of nickel and cobalt. Thus, the second metal-rich layer 4further enriched in nickel and cobalt is formed in the surface layerportion of the first metal-rich layer 3.

FIG. 5 shows the proportions of component elements (proportion, in atm%, of each component element in the entire composition of the layer) inthe first metal-rich layer 3 and the second metal-rich layer 4 of thehydrogen absorbing alloy powder, as determined by energy dispersiveX-ray analysis (EDX) using a field emission scanning transmissionelectron microscope (FESTEM).

The proportion of nickel (Ni) and the proportion of cobalt (Co) are bothgreater in the second metal-rich layer 4 than in the first metal-richlayer 3. This indicates that the second metal-rich layer 4 furtherenriched in nickel and cobalt is formed in the surface layer portion ofthe first metal-rich layer 3.

Thus, the first metal-rich layer 3 formed by the reduction treatmentwith high-temperature hydrogen and enriched in nickel and cobalt istreated with an acid to form the second metal-rich layer 4 which isfurther enriched in nickel and cobalt. The surface treating process ofthe invention therefore gives higher electrochemical catalytic activityto the hydrogen absorbing alloy electrode than the prior art wherein anacid treatment only is conducted.

FIGS. 7 and 8 show the results of tests conducted to substantiate theadvantage of the surface treating process of the present invention.

Described below are preparation of a test device, test method and testresults.

(1) Preparation of hydrogen absorbing alloy powders

Mm (a mixture of rare-earth elements), Ni, Co, Al and Mn (elementalmetal with a purity of 99.9%) were mixed together in the mole ratio of1.0:3.1:1.0:0.3:0.6, and the mixture was melted in an arc meltingfurnace having an argon atmosphere and thereafter allowed to coolspontaneously to obtain an ingot of hydrogen absorbing alloy representedby the formula MmNi_(3.1)CoAl_(0.3)Mn_(0.6). The ingot was mechanicallypulverized in the air to obtain a hydrogen absorbing alloy powder(untreated alloy powder 1) adjusted to a mean particle size of 80micrometers.

A hydrogen absorbing alloy powder (untreated alloy powder 2) was alsoprepared with the same composition and mean particle size by the gasatomizing process.

(2) Preparation of alloy powders by reduction

Untreated alloy powders 1 and 2 were placed into respectiveheat-resistant pressure-resistant containers of stainless steel andheated at varying temperatures of 50° C. to 950° C. after evacuation,hydrogen gas was then introduced into the containers to 1.2 atm, and thepowders were held in this state for 30 minutes to obtain alloy powdersreduced with high-temperature hydrogen.

(3) Preparation of alloy powers treated with acid

Untreated alloy powders 1 and 2, and the reduced alloy powders were eachimmersed in a 0.5N hydrochloric acid solution (room temperature) at a pHof 0.3 for 2 hours, followed by suction filtration, washing with waterand drying to obtain acid-treated alloy powders.

(4) Preparation of alloy powders treated with alkali

Untreated alloy powders 1 and 2, and the reduced alloy powders were eachimmersed in a 30 wt. % aqueous solution of potassium hydroxide (80° C.),which is for use as an electrolyte, for 2 hours, followed by suctionfiltration and drying to obtain alkali-treated alloy powders.

(5) Preparation of alloy electrodes

A 0.5 g quantity of each of various hydrogen absorbing alloy powdersthus prepared was admixed with 0.1 g of PTFE, the mixture was applied toan expanded nickel porous body serving as a conductive substrate, andthe body was then pressed at 1200 kgf/cm² for shaping, whereby ahydrogen absorbing alloy electrode was prepared in the form of a diskwith a diameter of 20 mm.

(6) Assembly of test cells

The electrodes thus obtained were used as test electrodes (negativeelectrodes) to assemble test cells like the one shown in FIG. 6.

As illustrated, the test cell has arranged in an insulating closedcontainer 21 of polypropylene a test electrode 22 which is the hydrogenabsorbing alloy electrode to be tested, a sintered nickel electrode 23in the form of a hollow cylinder and having a sufficiently greaterelectrochemical capacity than the test electrode 22, and a sinterednickel reference electrode 24 in the form of a plate. The nickelelectrode 23 is supported by the lower end of a positive electrode lead26 connected to the top wall 25 of the closed container 21. The testelectrode 22 is vertically supported by the lower end of a negativeelectrode lead 27 connected to the top wall 25 of the container 21, andis accommodated inside the nickel electrode 23 centrally thereof.

The positive electrode lead 26 and the negative electrode lead 27 extendthrough the top wall 25 of the closed container and are exposed to theoutside and connected to a positive terminal 28 and a negative terminal29, respectively. The test electrode 22 and the sintered nickelelectrode 23 are held immersed in an alkaline electrolyte (30 wt. %aqueous solution of potassium hydroxide). The closed container 21 isfilled with nitrogen gas in a space above the alkaline electrolyte,whereby the test electrode 22 is subjected to a predetermined pressure(5 atm). Connected to the center portion of the top wall 25 of theclosed container 21 is a relief pipe 32 equipped with a pressure gauge30 and a relief valve 31 for preventing the internal pressure of thecontainer 21 from increasing above a predetermined value.

(7) Assembly of alkaline batteries

Each hydrogen absorbing alloy powder and a 5 wt. % aqueous solution ofPEO (polyethylene oxide) were mixed together in the ratio of 100:20 byweight to prepare a paste, which was applied to opposite surfaces ofpunching metal (conductive substrate) plated with nickel, followed bydrying at room temperature and cutting to a predetermined size, toprepare a hydrogen absorbing alloy electrode. A nickel-hydrogen battery(1000 mAh in battery capacity) of AA size and of the type wherein thepositive electrode is dominant shown in FIG. 2 was then assembled usingthe electrode as the negative electrode. A sintered nickel electrode wasusable as the positive electrode, an alkali-resistant nonwoven fabric asthe separator, and a 30 wt. % aqueous solution of potassium hydroxide asthe electrolyte.

(8) Charge-discharge cycle test

At room temperature, each test cell was charged at 50 mA/g for 8 hours,then held at rest for 1 hour, subsequently discharged at 50 mA/g to afinal discharge voltage of 0.9 V and thereafter held at rest for 1 hour.This charge-discharge cycle was repeated, and the discharge capacity(mAh/g) was measured every cycle.

In the case of the alkaline batteries, each battery was charged withcurrent of 0.2 C for 6 hours and thereafter discharged at current of 0.2C to 1.0 V at room temperature to determine the initial dischargecapacity (discharge capacity of the first cycle).

(9) Measurement of electric resistance value

Each hydrogen absorbing alloy powder was checked for electric resistancevalue under the conditions of mean particle size of 35 micrometers,pressure of 350 kgf/cm², test jig inside diameter of 12 mm and powderweight of 5 g.

(10) Test results

FIGS. 7 and 8 shows the results of the test.

FIG. 7 shows the initial discharge capacity (discharge capacity 1) ofeach test cell, and the initial discharge capacity (discharge capacity2) of each alkaline battery. Alloy electrodes A1 to A6 are thoseprepared from the hydrogen absorbing alloys subjected to the reductiontreatment with high-temperature hydrogen at 300° C. Alloy electrodes B1to B6 are those obtained from the hydrogen absorbing alloys not treatedfor reduction. Alloy electrodes A1 to A6 which are treated for reductionare 285 mAh/g to 299 mAh/g in discharge capacity 1 and 820 mAh to 865mAh in discharge capacity 2, whereas alloy electrodes B1 to B6 which arenot treated for reduction are 170 mAh/g to 246 mAh/g in dischargecapacity 1 and 580 mAh to 675 mAh in discharge capacity 2. Thus, thealloy electrodes are greater in both discharge capacities and morehighly activated initially when treated for reduction than otherwise.

Alloy electrodes A1 to A6 treated for reduction include thoseacid-treated or alkali-treated after the reduction treatment, and thosetreated neither with acid nor with alkali after the reduction.Acid-treated alloy electrodes A2 and A5 are 295 mAh/g and 299 mAh/g,respectively, in discharge capacity 1, and 860 mAh and 865 mAh,respectively, in discharge capacity 2. Alkali-treated alloy electrodesA3 and A6 are 290 mAh/g and 296 mAh/g, respectively, in dischargecapacity 1, and are both 855 mAh in discharge capacity 2. On the otherhand, untreated alloy electrodes A1 and A4 are 285 mAh/g and 292 mAh/g,respectively, in discharge capacity 1, and 820 mAh and 840 mAh,respectively, in discharge capacity 2. Thus, the acid-treated oralkali-treated alloy electrodes are greater in both discharge capacities1 and 2.

Accordingly, although the reduction treatment with high-temperaturehydrogen, even when singly conducted, results in great dischargecapacities as described above, further enhanced effects are availablewhen the acid treatment or alkali treatment, preferably acid treatment,is carried out after the reduction treatment.

FIG. 7 reveals that the alloy electrode of the invention (A2 or A3)subjected to the reduction treatment and the acid treatment is givenhigher activity than the conventional alloy electrode (B2 or B3) whichis subjected to the acid treatment only.

Alloy electrodes A1 to A3 and B1 to B3 are prepared from the alloypowder which is obtained by mechanically pulverizing an ingot made by anargon arc furnace, while alloy electrodes A4 to A6 and B4 to B6 areprepared from the alloy powder obtained by the gas atomizing process.These two groups of electrodes will be compared in discharge capacities1 and 2, as divided in two cases depending on whether the alloy istreated for reduction with high-temperature hydrogen or otherwise. Inthe case where no reduction treatment is conducted, alloy electrodes B1to B3 are 222 mAh/g to 246 mAh/g in discharge capacity 1 and 620 mAh to675 mAh in discharge capacity 2, while alloy electrodes B4 to B6 are 170mAh/g to 221 mAh/g in discharge capacity 1 and 580 mAh to 620 mAh indischarge capacity 2. Thus, alloy electrodes B1 to B3 prepared with useof the argon arc furnace are greater in both discharge capacities 1 and2.

In the case where the reduction treatment is conducted, on the otherhand, alloy electrodes A1 to A3 are 285 mAh/g to 295 mAh/g in dischargecapacity 1 and 820 mAh to 860 mAh in discharge capacity 2, while alloyelectrodes A4 to A6 are 292 mAh/g to 299 mAh/g in discharge capacity 1and 840 mAh to 865 mAh in discharge capacity 2. Thus, alloy electrodesA4 to A6 prepared by the gas atomizing process are greater in bothdischarge capacities 1 and 2.

Accordingly, it is advantageous to prepare the alloy powder by the gasatomizing process in respect of the initial activity in the case wherethe reduction treatment is conducted with high-temperature hydrogen.

Furthermore, alloy electrodes A1 to A6 treated for reduction are lowerthan alloy electrodes B1 to B6 not treated for reduction in theresistance value of the powder as measured under the conditions ofparticle size of 35 micrometers, pressure of 350 kgf/cm², measuring jiginside diameter of 12 mm and powder weight of 5 g. This substantiatesthat the alloy particles are formed, in their surface layer portionswith a metal-rich layer having higher nickel and cobalt contents than inthe alloy electrode not treated for reduction with high-temperaturehydrogen.

FIG. 8 shows the results obtained by measuring discharge capacities 1and 2 of alloy electrodes which were prepared from the alloy powder madewith use of an argon arc furnace and the alloy powder obtained by thegas atomizing process after subjecting the powders to the reductiontreatment with high-temperature hydrogen at varying temperatures of 50°C. to 950° C.

The alloy electrodes prepared with use of the argon arc furnace are asgreat as at least 284 mAh/g in discharge capacity 1 and at least 820mAh/g in discharge capacity 2 when the temperature for the reductiontreatment is in the range of 100° C. to 900° C.

The alloy electrodes prepared by the gas atomizing process are as greatas at least 288 mAh/g in discharge capacity 1 and at least 835 mAh/g indischarge capacity 2 when the temperature for the reduction treatment isin the range of 100° C. to 900° C.

For the alloys thus used for testing, it is suitable that thetemperature for the reduction treatment be in the range of 100° C. to900° C. under the conditions of pressure of 1.2 atm and time period of30 minutes regardless of whether the argon arc furnace or the gasatomizing process is used. If the reduction treatment temperature is atleast 100° C., the equilibrium hydrogen pressure of the hydrogenabsorbing alloy rises to suppress the absorption of hydrogen by thealloy.

INDUSTRIAL APPLICABILITY

The hydrogen absorbing alloy power embodying the invention is suitableas a material for electrodes of metallic oxide-hydrogen batteries, forexample, as a material for the negative electrodes of nickel-hydrogenbatteries.

What is claimed is:
 1. A hydrogen absorbing alloy electrode comprisingan electrically conductive substrate having applied thereto a hydrogenabsorbing alloy powder, the electrode being characterized in that thehydrogen absorbing alloy powder contains nickel and cobalt, the alloyparticles (1) being formed each in a surface layer portion thereof witha metal-rich layer (3) enriched in metals by a reduction treatment withhydrogen, the metal-rich layer (3) being surface-treated with an acid oralkaline treating liquid.
 2. A hydrogen absorbing alloy electrodeaccording to claim 1 wherein the acid treating liquid is a hydrochloricacid solution.
 3. A hydrogen absorbing alloy electrode according toclaim 1 wherein the alkaline treating liquid is at least one aqueoussolution selected from among an aqueous solution of KOH, aqueoussolution of NaOH and aqueous solution of LiOH.
 4. A metallicoxide-hydrogen battery wherein a hydrogen absorbing alloy electrode isused which comprises an electrically conductive substrate having appliedthereto a hydrogen absorbing alloy powder, the metallic oxide-hydrogenbattery being characterized in that the hydrogen absorbing alloy powdercontains nickel and cobalt, the alloy particles (1) being formed each ina surface layer portion thereof with a metal-rich layer (3) enriched inmetals by a reduction treatment with hydrogen, the metal-rich layer (3)being surface-treated with an acid or alkaline treating liquid.
 5. Ametallic oxide-hydrogen battery according to claim 4 wherein the acidtreating liquid is a hydrochloric acid solution.
 6. A metallicoxide-hydrogen battery according to claim 4 wherein the alkalinetreating liquid is at least one aqueous solution selected from among anaqueous solution of KOH, aqueous solution of NaOH and aqueous solutionof LiOH.